This invention relates generally to video viewing of an image both in real-time and from a recording. More particularly, it relates to the synchronization of the triggering of a pulsed light source to video viewing and recording devices and electronic shutters and to the suppression of ambient background light relative to the pulsed light in the same image frame for such devices. The invention triggers the light source at intervals dictated by the video camera signal or may, in the alternative, trigger the light source in response to an external voltage pulse.
There is a generally recognized need for the video viewing of various pulsed light sources, in particular pulsed laser or lidar sources, both in real-time and from a recording. The light pulse width is typically short relative to the time it takes to produce one television screen field (i.e.  less than {fraction (1/60)} second). Two fields make up a television frame, which is one complete image. The two fields are virtually identical in terms of image content and have been provided in standard television systems to reduce apparent flicker to the observer. Capture of the pulsed light at a specific time in the television frame is random. While this randomness does not interfere with direct viewing in real-time or continuous viewing of a recorded image from a standard video cassette recorder (xe2x80x9cVCRxe2x80x9d), continuous viewing of one image, i.e., employing the xe2x80x9cpausexe2x80x9d function on a video tape recorder, does not always show the light pulse. It xe2x80x9cdisappearsxe2x80x9d when the tape is paused. This is because the pulsed light can fall randomly within either of the two video fields that make up the video frame. The system employed on virtually all standard VCR""s presents the entire frame for viewing under normal running conditions. However, in the xe2x80x9cpausexe2x80x9d mode, only one field, either the first or second depending on the particular model VCR, is consistently chosen for display of the image. This design is employed to avoid jitter when viewing a xe2x80x9cpausedxe2x80x9d recording with the typical spinning record head used in VCR""s. It is impossible to consistently establish the synchronization of the light pulse to the same video field required for the xe2x80x9cpausexe2x80x9d mode while at the same time continuously viewing the real-time image capture process on a monitor. The entire frame is presented during real-time and there is no indication during which field the pulse of light is emitted.
Historically, attempts were made to manually adjust the repetition rate of the pulsed light source to match that of the proper field of the video device, providing a satisfactory solution that related to the precision of establishing and maintaining the relative frame and pulse rates. The process was tedious because the image had to be recorded and then the tape rewound and viewed in the pause mode to confirm capture of the pulsed light in the proper field. The light pulse system timing and the camera system video rate could also drift over time relative to each other, disrupting the synchronization. Since at least half the light pulses, on the average, would be in the correct field, the problem was ignored, particularly because the light pulse could be seen at all times with continuous viewing. There is, however, a need for a simple system to allow for synchronization of the light pulse to the proper video field.
In addition, there has been a long standing need to provide a system that will allow predictable capture of a pulsed light source by video cameras that incorporate electronic shutters. These shutters arc in most cameras and are typically used to reduce image blurring in a moving scene. At best, with the present approach, there is only random capture of pulsed light. This occurs when a pulse is of shorter duration than, and coincident with, the shutter open time. Unlike the case without a shutter, the pulsed light will only be seen occasionally in a non-synchronizcd system operating continuously.
Furthermore, in viewing a video recording of a weak pulse of light in a high ambient light background, some suppression of the background light is needed or desired. However, continuous suppression of the background light would normally interfere with viewing the pulsed light in the context of the background. For instance, while viewing a laser spot for spatial positioning on a bright surface, it is necessary to see the surface. The prior art does not teach how to suppress the background without also suppressing the pulsed light image. In most circumstances, it is desirable to retain background information while seeing the pulsed light source. There is a need for a simple system that would allow examination, in the same frame, of both the background and a weak pulse of light in that background.
In addition, it is not apparent that any attempt has been made historically to have manual or external voltage pulses to allow arbitrary, yet still synchronized, triggering of light pulses for video viewing, recording, and background light control. Arbitrary triggering would include xe2x80x9cone-shotxe2x80x9d manual triggering or external triggering sources such as pulse generators, including both traditional, repetitive pulse sources as well as irregular sources of pulses.
Accordingly, it is an object of this invention to provide an improvement in the video viewing, both in real-time and from a recording, of pulsed light sources.
It is a further object of this invention to provide an apparatus and method for synchronizing a pulsed light source to a video camera so that the pulse can predictably be seen during the pause mode of a VCR.
It is another object of this invention to provide an apparatus and method for synchronizing a pulsed light source to the opening of the electronic shutter employed in a video camera.
It is yet another object of this invention to provide an apparatus and method for the suppression of background ambient light when imaging pulsed light without effecting the intensity of the pulsed light itself, and yet allowing the background to be viewed.
It is still another object of this invention to provide for arbitrary triggering of a pulsed light source in such a manner that the light pulse is synchronized to the video signal for video recording and viewing and electronic shutter control, if available.
Briefly, the foregoing objects are obtained through interface electronics which controls the triggering or firing of each pulse of light from a triggerable pulsed light source such as strobes or, in particular, a pulsed laser. This control is obtained by deriving a synchronization pulse from a camera video signal. Further, the interface electronics may control the electronic shutter on a camera, where the shutter speed on the camera is controlled on a frame-by-frame basis. The term xe2x80x9cshutter speed,xe2x80x9d in this instance, means the duration of time that the shutter remains open, effecting the exposure. This allows the shutter speed to be varied on each frame. This feature is employed to obtain suppression of the background ambient light in the frame where the light pulse is captured. The invention further allows said synchronization to be continuous, i.e., the light pulse being triggered at regular, arbitrary intervals, or at manual, i.e., single triggering events implementted by, e.g., a button-type switch or other arbitrary external occurring trigger sources.